An attempt to form a new government of national unity for El Salvador appeared on the point of collapse today after nearly a week of negotiations among five rival political parties. Discussions recessed for Easter with members of the ruling Christian Democratic Party talking openly about refusing to join a coalition government and moving into opposition instead.

This would mean that the new government to be formed by the constituent assembly elected March 28 would be controlled by two rightwing parties that have indicated in the past that they oppose a number of the economic and social reforms that the Christian Democrats and U.S. policymakers have said are vital in fighting El Salvador's guerrillas.

The Christian Democrats and the four rightist parties have conducted this week's negotiations with a shared assumption that would also be troubling for Reagan administration officials who have been promoting the constituent assembly as a major advance for democratic principles here. That assumption is that the party leaders would make policy decisions this week behind closed doors, and the assembly would be essentially a rubber stamp legislature for those decisions.

The Reagan administration has put its full weight behind an effort to bring the Christian Democrats into the next government, in part to gain continued congressional support for the proposed $165 million in new U.S. military and economic aid to fight the leftist guerrilla movement here.

The Christian Democratic leaders who gave the pessimistic assessment of the negotiations today acknowledged that if the party becomes the formal opposition in the assembly, it will have to fight from a minority position with the dominant right wing in order to preserve the social and agricultural reforms of its 29 months of government. Within a national unity government, which would include them, the Christian Democrats would exert a moderating influence on policy, but the policy would be decided behind closed doors and not on the floor of the assembly, they said.

"Ideally all issues would be decided before the assembly meets," said a high party official who asked not to be named, "and it would agree to take the steps the government had decided were necessary."

Asked how that squared with the traditional view of open legislative debate on major issues, he smiled. "We are a country at war," he said, referring to the leftist guerrilla uprising still raging in the countryside. "We cannot fight on so many fronts at once."

However, he said, the negotiators have not been able to agree, for example, on how the government would go about "perfecting" the 1980 land reform program as the rightist parties promised the voters they would do. Such issues, he added, were too important to the Christian Democrats to be left to the legislature.

Sources within the National Conciliation Party, which ran El Salvador for the landed oligarchy from 1961 to 1979, had a slightly different version of the hangup in the talks. "We are the party with a platform," said one of the party directors. "That is the basis for our discussions." He complained that the negotiators would occasionally reach what they thought was an agreement on an issue and then the Christian Democrats would retire for consultations and come back having changed their minds.

He said the parties were negotiating which of them would have control over which sectors of the political panorama: the economy, foreign affairs, public works, health care and so on. The legislature, he said, "would naturally follow" directions on an issue made by the party in control of that area. "The PDC Christian Democrats lost the election, and they must realize that they cannot control the new government," he said. "They will participate, but they cannot control it."

The Christian Democrats won 24 seats in the 60-seat legislature, while National Conciliation got 14, the power broker's position, and the far-right Nationalist Republican Alliance (known as ARENA) took 19. Democratic Action has two seats and the Popular Salvadoran Party has one. Democratic Action leaders have said privately that if the Christian Democrats go into opposition, they will too, even though their combined vote would still be well short of a majority. The government will be a provisional one, empowered to name an executive, write a new constitution and set up presidential elections sometime in the future.

President Jose Napoleon Duarte, a Christian Democrat virtually certain to hold no post whatever in the new government, told eight visiting U.S. members of Congress today that he is pessimistic about the formation of a national unity coalition.

"He talked about readying himself and his party for a role in the opposition," Rep. Michael D. Barnes (D-Md.) told reporters . Barnes is chairman of the Latin America subcommittee of the House Committee on Foreign Affairs.

Duarte also told reporters before the congressmen arrived that he would prefer that the existing military-civilian junta remain in power than that ARENA and National Conciliation run the country. He told NBC News that he had been told unofficially that the rightists had selected a wealthy ARENA lawyer, Antonio Rodriguez Porth, to be president in the provisional government, and that a National Conciliation retired Army colonel, Roberto Escobar Garcia, would be named vice president. Party spokesmen denied both claims, saying nothing is yet certain.

"The right wing has used democracy to seize power," Duarte told CBS.

Duarte has repeatedly warned the nation that the Christian Democrats in opposition would be formidable, accustomed as it is to the ways of power and familiar from 20 years of opposing military governments with the pitfalls of being on the outs. The armed forces, which installed Duarte as president of the current ruling three-man junta in 1980, have backed the reforms his government made and would not look kindly upon their wholesale dismantling, he has said.

The other Christian Democratic official agreed. "The power of the legislature will be determined not by law, but by the realities of the country," he said. "In fact there are four political powers: the three main parties and the armed forces."

Military officials have said repeatedly that they very much want a united political front against the guerrillas, and no one here thinks they would stand idle if a civilian government falls to ineffective squabbling, even if the squabbling is in a fully democratic pattern of open debate.

Majority Leader James C. Wright Jr. (D-Texas) said the congressional delegation, which is here for only 24 hours, brought no message from Congress to El Salvador's politicians but noted that Congress feels that a broad-based government would be "far preferable" to one controlled by the right wing.

The congressmen were scheduled to leave Friday morning for Nicaragua, Costa Rica and Jamaica.